Are single frequency detectors going out of style?

[teacup]

I am searching for a detector to do gold prospecting and find tiny gold pieces.. I am looking at the Gold Bug 2, Kruzer, and Goldmaster 24k. I read in a forum that in the due to technology advancing that single frequency detectors wont be popular in the future... I would like my detector to retain it's value over the years in case i decide to sell it or swap it out for another one in the future. Will the single frequency ones lose value as the multi frequency detectors start becoming more affordable? Thanks